1. Field of the Invention
This invention relates to an image sensing apparatus having image sensing means arranged to have the sensitivity thereof controllable.
2. Description of the Prior Art
In taking a picture of an object with an image sensing apparatus, the sensitivity of currently available image sensors is insufficient for an object of low brightness or luminance. To solve this problem, it has been the practice either to use artificial illumination or to raise the apparent sensitivity of the image sensor by electrically increasing the gain of a signal processing system. The details of this latter arrangement of electrically raising the image sensor sensitivity are as follows: Conventional video cameras include cameras of the kind provided with an ALC circuit for automatically controlling the aperture of an iris according to the output of the image sensor. In the case of the video camera of this kind, the gain of the signal processing circuit is arranged to be automatically increased in the event that the mean value of the output level of the image sensor is lower than an adequate value with the iris opened to its maximum aperature.
In the case of the arrangement to have the gain automatically increased, however, the S/N ratio of a video signal sometimes suddenly changes halfway through the image sensing operation apart from the intention of the operator. In other words, with the gain increased, the internal noise of the image sensor is also amplified resulting in a sudden degradation of picture quality. However, in most cases, the operator is unaware of the occurrence of this sudden degradation, because, in the case of a compact electronic view finder (EVF) arrangement, it is nearly impossible to discriminate such a change in the S/N ratio.
Further, during recent years, there has been a development in the field of image sensing apparatuses to devise arrangements for sensing a still picture image. It is necessary for a still picture image sensing apparatus to accomplish the sensitivity control and exposure control within a short period of time. Therefore, a servo control arrangement or the like which takes a long time is inadequate for that purpose. Furthermore, a continuous control arrangement for sensitivity control is also inadequate both in terms of time and structural arrangement. More specifically, an arrangement for continuously controlling the sensitivity requires, among others, digital-to-analog (D/A) conversion of a control signal, which complicates the structural arrangement thereof. Then, any attempt to avoid such complexity by dispersively accomplishing the sensitivity control in accordance with the conventional arrangement, results in a long pursuing time required by the exposure control servo device such as an iris, etc.